The present invention relates to open cell foam used as ink reservoirs in ink printing pens. More specifically, the present invention relates to a method and apparatus whereby pressure is varied between plasma treatment cycles to produce an outdiffusion of reaction products from the foam. This outdiffusion together with the indiffusion of reactants into the foam to provides more uniform surface treatment throughout the interior surfaces of the foam thereby increasing foam bulk wettability characteristics.
Ink jet printers employ ink reservoirs to supply a printhead with ink. An ink reservoir consists of foam housed within a pen cartridge, the foam having been permeated with ink. The foam absorbs and retains the ink, thereby enabling a printer to control the rate at which ink flows to the printhead.
The process of ink jet printing requires that a continual supply of ink be available to the printhead. In thermal ink jet printers, the printhead comprises a plurality of tiny nozzles formed in a nozzle plate. Underneath each nozzle is a firing chamber which is commonly feed with ink from a plenum connected to the ink reservoir. A microprocessor in the printer provides a signal that directs the expulsion of ink through a nozzle so that the ink forms a droplet. The trajectory of the droplet is such that the droplet strikes the paper at a precise target. The precision required in ink jet printing makes the ink reservoir's ability to efficiently, reliably, and predictably supply ink to the printhead nozzles critical.
Therefore, the foam within the ink reservoir must effectively retain ink to prevent unintentional seepage of the ink out of the printhead nozzles. This unintentional seepage of ink from the cartridge is sometimes referred to as "drool" which, in the case of multi-color printing, can result in inadvertent color mixing if ink emanating from a nozzle associated with one color is pulled into a nozzle associated with another color. This inadvertent color mixing results in a reduction of the output image quality. Additionally, the foam must supply ink as required by the printhead. Lastly, the foam must accomplish these goals without adversely effecting the quality of the ink.
As manufactured, most organic polymer based foam is unable to meet these goals. Untreated polyurethane foam is hydrophobic and therefore does not readily absorb aqueous based inks. One method used to prepare foam to serve as an ink reservoir involves prewetting the foam prior to loading it with ink. Prewetting the foam reduces its hydrophobicity so that the foam absorbs rather than repels the ink. A method for prewetting foam is discussed in U.S. Pat. No. 5,467,117 entitled "Addition Of Alcohol To Prewet Solutions To Enhance And Accelerate Wetting To Hydrophobic Foams For Application To Ink Jet Pens " to Evans et al assigned to the assignee of the present invention and incorporated herein by reference.
Plasmas have been used to modify the surface structure of polymer solids as discussed in "Techniques and Applications of Plasma Chemistry", edited by John R. Hollahan and Alexis Bell, published by John Wiley and Sons, 1974. These surface modifications brought about by exposure to a plasma include changes to the surface wettability, molecular weight of a surface layer and the chemical composition of the surface. However, as pointed out by Hollahan the effects of the plasma treatment are confined to the surface layer and therefore the bulk properties of the material remain unchanged. (See page 117).
U.S. Pat. No. 5,447,756 entitled "Method Of Forning Surface Treated Applicators" to Kamen discloses a method whereby a plasma surface treatment of cosmetic applicators such as mascara brushes, makeup brushes and sponge like applicators to improve the hold, wettability, pickup, laydown, release, and application. The plasma treatment process is described as a grafting process whereby a substrate layer is grafted to the cosmetic applicator surface. The cosmetic applicator is used to apply paint, powder, make-up or nail enamel to a surface.
Reticulated or open cell foam has some very unusual attributes which make it not well suited to the plasma treatment techniques previously used for improving wettability. For example, open cell foam has an interior which is a dense matrix structure with confined passages deep within the foam. The surface area of the foam is therefore very large for a given unit mass. Frequently, the foam used in ink storage applications is compressed using thermal compression techniques such as felting. Highly felted foam tends to have a structure that is more dense and more confined than unfelted foam which further adds to the problem of treating surfaces deep within the foam matrix.
Previously used surface treatment techniques are not well suited for treating foam which has confined passages deep within the foam. One problem associated with plasma treatment of foam is that there is limited radical generation inside of the foam. Radicals are generated mainly by collision processes with electrons, ions, or other species. The confined passages within the foam provide a smaller mean free path than the mean free path required to form radicals. The very small mean free path makes it much more likely the electrons, ions and other species will collide with a foam surface near the periphery of the foam than deep inside the interior of the foam. Therefore, within the foam matrix there are few of these excited elements from which to form radicals.
Another problem associated with plasma treatment of foam is that there is little diffusion of radicals into the foam. Since the mean free path of radicals is larger than the foam passage dimensions, radicals rarely reach the interior of the foam without first colliding with, and gettered by the peripheral surfaces of the foam 12. Finally, there is little diffusion of reaction products out of the foam. This limited outdiffusion of reaction products from the foam 12 results in higher concentrations of reaction products which slow the reaction rate of radicals within the foam 12. Therefore, conventional plasma treatment of foam treats only surfaces along the periphery of the foam and provides little improvement to wettability characteristics of interior surfaces of the foam matrix.
There is a continuing need for improved methods for improving the wettability characteristics of the foam interior surfaces. This treatment should be capable of treating not only the surfaces near the periphery of the foam but also surfaces deep within the dense matrix of the foam. This treatment should increase the wettability throughout the interior surfaces or foam bulk to prevent areas of increased capillary or capillary gradient which can result in the stranding of ink within the foam. Finally, this treatment should be cost effective to reduce the overall manufacturing costs of the ink cartridge.